Here are the steps to resolve this issue.
- Click on Settings
- Click on Manage Email Accounts
- Edit the email account that is having issues
- Click on the Outgoing Mail tab
- Change the SMTP Port to be 587
- Change the Type of Encryption to be TLS
- Click the save button
If the above steps do not resolve your issue please refer to the tutorial in the following link for other reasons why this issue may be happening.